F-340 Experts - Altitude Screw Adjustment Question

Teds

Teds

Active Member
Joined
Jan 14, 2017
Threads
2
Messages
82
Lawn-Boy Rocky said:
Fishnuts2,
Thanks for the info. So it adjusts opposite of what conventional wisdom might suggest. Very counter intuitive, but effective once folks know.

CW = richer and CCW = leaner.

I'll stay on the slightly rich side to keep this 1980 UTILITY model happy.:smile:

Rocky
Click to expand...

No, clockwise leans it out. Counterclockwise richens the fuel mixture! It's not rocket surgery.
 
L

Lawn-Boy Rocky

Member
Joined
May 22, 2019
Threads
7
Messages
40
Teds said:
No, clockwise leans it out. Counterclockwise richens the fuel mixture! It's not rocket surgery.
Click to expand...

So, what I see here is two guys (fishnuts and Ted) with two opposite answers to my original post.
This is why I'm still hanging in here, trying to get to the bottom of this.
Not sure which carb Ted's info (7) relates to, but I do not have a high and low speed "setting" on my UTILITY plastic carb.
Rocky
 
J

jp1961

Well-Known Member
Joined
Jul 22, 2013
Threads
39
Messages
785
Hi Rocky,

Are you referring to a small plastic lever on the back of shroud? If so, then that's actually adjusting the governor spring tension, not the carburetor.

The mixture needle valve would be on the carb itself.

Regards

Jeff
 
L

Lawn-Boy Rocky

Member
Joined
May 22, 2019
Threads
7
Messages
40
Thanks to all for your input re: this screw adjustment.
I have seen various opinions on the various LB carbs.:wink:
Rocky
 
Teds

Teds

Active Member
Joined
Jan 14, 2017
Threads
2
Messages
82
The beauty here guys, we don't need to rely on opinion, this isn't me talking, we can simply read the manual. It's right there in black & white, straight from the horses mouth.

I haven't pored over every single jot and tittle in the manual at least not recently, but I'm not aware of any carburetor altitude screws on any model that work contrary to CW=Leaner CCW=Richer AFR. The carburetors themselves are VERY simple construction, when you take them apart the next time take a look at the innards and it will make sense.
 
tom3

tom3

Lawn Addict
Joined
Apr 9, 2018
Threads
25
Messages
1,579
I don't know much about it but on my old F engine with metal Walbro carb I can take the screw clear out and it runs the same. Seat it and turn it out 3/4 turn and it runs the same - but dies in a half minute or so. I just can't see this as a normal mixture adjustment.
 
Teds

Teds

Active Member
Joined
Jan 14, 2017
Threads
2
Messages
82
tom3 said:
I don't know much about it but on my old F engine with metal Walbro carb I can take the screw clear out and it runs the same. Seat it and turn it out 3/4 turn and it runs the same - but dies in a half minute or so. I just can't see this as a normal mixture adjustment.
Click to expand...

Well it would make more sense that probably something is defective with that particular carburetor versus the manual has it wrong?
 
tom3

tom3

Lawn Addict
Joined
Apr 9, 2018
Threads
25
Messages
1,579
I've suspected that since I got the mower, but a new carb is real hard on the bank account - when you can find one. And a 30 year old steel deck LB isn't worth a whole lot anyway. Wish the Chinese would get their act together and produce an $8 replacement.
 
Teds

Teds

Active Member
Joined
Jan 14, 2017
Threads
2
Messages
82
Hm. They are so simple, it can't really be any weird stuff. Carburetors are designed and calibrated around a very specific fuel column height that is maintained in the bowl for siphon action through the jet. The fuel height is set indifectly through the float height adjustment. Too low of an average fuel level tends to run lean, too high it will run rich. There isn't really any way to check that on these. Automobile carburetors had some ingenious mechanisms, Holley float height for example was adjustable on the fly without disassembly, and resulting fuel level observed through a weephole in the side of the carb.
 
You must log in or register to reply here.

Footer Links 1

Top